Recently, an image forming apparatus of an electrophotographic system, such as a laser printer, a digital copying machine or a laser fax, includes a finisher to stitch a sheet bundle. The finisher of the related art includes a stapler to stitch a sheet bundle. The finisher of the related art includes a sheet discharge mechanism to discharge a sheet bundle. The finisher including the sheet discharge mechanism discharges the sheet bundle by a roller and a bundle hook of a drive belt.
However, the finisher can drive the bundle hook of the drive belt only in one direction. There is a problem that if the finisher continues to drive the bundle hook in one direction and the bundle hook returns to a home position of the bundle hook, the bundle hook collides with sheets or sheet bundles which the sheet discharge mechanism discharges and stacks, and an error or a sheet jam is liable to occur.